Electronic microcomponent of the variable capacitor or microswitch type, and process for fabricating such a component

ABSTRACT

Process for fabricating electronic components, of the variable capacitor or microswitch type, comprising a fixed plate ( 1 ) and a deformable membrane ( 20 ) which are located opposite each other, which comprises the following steps, consisting in:  
     depositing a first metal layer on an oxide layer ( 2 ), said first metal layer being intended to form the fixed plate;  
     depositing a metal ribbon ( 10, 11 ) on at least part of the periphery and on each side of the fixed plate ( 1 ), said ribbon being intended to serve as a spacer between the fixed plate ( 1 ) and the deformable membrane ( 20 );  
     depositing a sacrificial resin layer ( 15 ) over at least the area of said fixed plate ( 1 );  
     generating, by lithography, a plurality of wells in the surface of said sacrificial resin layer;  
     depositing, by electrolysis, inside the wells formed in the sacrificial resin ( 15 ), at least one metal region intended to form the deformable membrane ( 20 ), this metal region extending between sections of the metal ribbon ( 10, 11 ) which are located on each side of said fixed plate ( 1 );  
     removing the sacrificial resin layer ( 15 ).

TECHNICAL FIELD

[0001] The invention relates to the field of microelectronics, andespecially to the field of microsystem components. It relates moreparticularly to microcomponents of the variable microcapacitor ormicroswitch type which integrate a membrane that can deform under theaction of electrostatic forces. It also relates to a particular processallowing such microcomponents to be obtained, which proves to be greatlyadvantageous over the existing processes.

PRIOR ART

[0002] It is known that the microcomponents of the microcapacitor ormicroswitch type have a fixed plate and a movable membrane which areseparated by a volume whose dimensions can vary, especially for examplewhen a continuous potential difference exists between the fixed plateand the deformable membrane.

[0003] By subjecting the fixed plate and the movable membrane to aparticular potential difference, it is thus possible to vary the nominalvalue of the capacitance according to the desired application.

[0004] In certain situations, it is also possible to ensure that themovable membrane moves sufficiently close to the fixed plate to makecontact. The component is then used as a microswitch.

[0005] Microcomponents of this type are generally obtained by usingtechniques associated with surface micromachining. Conventionally, sucha membrane is obtained by processes carried out at high temperature,above 400° C., and more generally above the temperatures that can bewithstood by a finished semiconductor without any risk of itsfunctionalities being too greatly modified.

[0006] A component typically consists of a stack of thin layers, withthe following conventional structure. A first layer constitutes thefixed plate of the capacitor. This layer may be made of polysilicon ofthe first level when the technologies used are compatible withsemiconductor processing methods. This polysilicon layer is deposited onan oxide or nitride layer. In the document Darrin J. Young and BernhardE. Boser “A Micromachine-Based RF Low-Noise Voltage-ControlledOscillator”, IEEE 1997 Custom Integrated Circuits Conference, May 1997,pp. 431-434, microcomponents were also described in which the fixedplate is made of aluminum.

[0007] The layer that has to provide the future volume between the fixedplate and the movable plate is made of a sacrificial material depositedon top of the layer forming the fixed plate by any process compatiblewith the type of microcomponent that it is desired to produce. Thus,this material may conventionally be silicon dioxide (SiO₂) or aderivative compound which will be removed by acid etching.

[0008] The upper layer, forming the deformable plate of themicrocomponent is conventionally made of polysilicon obtained by LPCVD(low-pressure chemical vapor deposition) and is doped sufficiently toreduce its resistivity. Certain techniques propose a conductive coatingplaced on top of the polysilicon in order to decrease the apparentresistivity thereof. This movable plate is anchored to the substratethrough vias, that is to say holes etched in the oxide and/or thesacrificial film. This movable and deformable plate may also be slightlytextured by features etched partially in the oxide or sacrificial filmbefore the latter is deposited. This texturing gives the lower face ofthe movable plate a certain relief and limits the area of contactbetween the fixed plate and the movable plate in the case in which theytouch. Thus, any bonding phenomena are avoided.

[0009] In order to dissolve the sacrificial layer by chemical etching,or any process allowing isotropic etching, it is essential for the layerconstituting the movable plate to also be pierced over its entire areain the form of regular and repeated patterns allowing the dissolvingsolution to pass.

[0010] A major drawback of this type of process is that the depositionof the polysilicon layer requires the use of a high-temperaturetechnique which is not compatible with deposition on semiconductorlayers whose functionalities run the risk of being significantlymodified or degraded by heat. It is therefore impossible with this kindof technique to produce microcomponents directly on existing integratedcircuits by a “post-processing” technique.

[0011] The invention therefore aims to overcome these various drawbacks.

SUMMARY OF THE INVENTION

[0012] The invention therefore relates to a process for fabricatingelectronic microcomponents of the variable capacitor or microswitchtype, comprising a fixed plate and a deformable membrane which arelocated opposite each other.

[0013] This process comprises the following steps, consisting in:

[0014] depositing a first metal layer of complex shape on an oxidelayer, said first metal layer being intended to form the fixed plate;

[0015] depositing a metal ribbon, forming a border, on at least part ofthe periphery and on each side of the fixed plate, said ribbon beingintended to serve as a spacer between the fixed plate and the deformablemembrane;

[0016] depositing a sacrificial resin layer over at least the area ofsaid fixed plate;

[0017] generating, by lithography, a plurality of wells in the surfaceof said sacrificial resin layer;

[0018] depositing, by electrolysis, inside the wells formed in thesacrificial resin, at least one metal region intended to form thedeformable membrane, this metal region extending between sections of themetal ribbon which are located on each side of said fixed plate;

[0019] removing the sacrificial resin layer.

[0020] In other words, the process according to the invention allowsproduction of deformable membranes produced by electrolysis, whichprocess is carried out at room temperature. This therefore allows themicrocomponent to be placed on various substrates, including integratedcircuits.

[0021] Thus, in a first family of applications, the process according tothe invention may be implemented using, as oxide layer, a quartz layerin order to form components incorporating only microcapacitors ormicroswitches.

[0022] In another type of application, the process may be implementedusing, as oxide layer, an oxide layer deposited on an integrated circuitso that the microcapacitors or microswitches may be placed directly ontop of the integrated circuit and can interact with functional regionsof the integrated circuit, thereby limiting as far as possible theinfluence of the connection system since these microcomponents arecloser to the integrated circuit. High integrated density is alsoachieved.

[0023] In practice, the first metal layer intended to form the fixedplate of the microcomponent is advantageously inserted into a recessformed in the oxide layer. In other words, the fixed metal plates may beobtained by a “damascene” metallization process. This allowsparticularly reliable components to be obtained, since they are strongand vibration-resistant. Furthermore, by virtue of the excellentflatness of the layers obtained by this process, it is possible tosuperpose several layers without building up topological irregularities.The subsequent operations are thus facilitated.

[0024] In practice, the first metal layer forming the fixed plateadvantageously includes an extension associated with a connection padmounted on or inside the oxide layer. This connection pad allows themicrocomponent to be linked either to the subjacent integrated circuitor to other parts of an electronic circuit.

[0025] According to another characteristic of the invention, the spacerribbon is present along the periphery of the fixed plate, on two opposedsides of the latter. The segments of this ribbon then serve to supportthe ends of the deformable membranes.

[0026] In practice, the spacer ribbon may consist of a continuous band,or even advantageously of a succession of individual segments, presentonly in the regions receiving the ends of the deformable membranes.

[0027] In practice, the sacrificial resin layer is advantageouslydeposited in such a way that it partly covers the peripheral spacerribbon. In this way, when the deformable membrane is deposited on top ofthe sacrificial resin layer, the region where it joins the spacer ribbonhas breaks in slope, facilitating flexure of the deformable membrane.

[0028] In practice, the process according to the inventionadvantageously also includes a step consisting in etching the oxidelayer in order to form one or more anchoring grooves intended toaccommodate part of the peripheral ribbon, or else part of the ends ofthe deformable membrane. This anchoring groove is located directlyoutside the peripheral ribbon, or else partly beneath the peripheralribbon. This groove accommodates part of the peripheral ribbon or elsepart of the membrane in order to ensure that it is deeply anchored inthe substrate, thereby increasing the robustness of the microcomponent.

[0029] In practice, it has been determined that the anchoring issatisfactory when the groove advantageously has a width about twice thethickness of the deformable membrane and that, complementarily, thedepth of the groove is more than one and a half times its width.

[0030] According to another characteristic of the invention, the processmay furthermore include a step consisting, after the fixed plate hasbeen deposited, in depositing a film of dielectric, intended to preventthe deformable membrane from bonding to the fixed plate. Thus, directcontact between the fixed plate and the deformable plate, which couldcause these two plates to bond together, and therefore damage themicrocomponent, is avoided. The value of the capacitance per unit areamay, furthermore, be improved if the dielectric constant of theadditional film is greater than that of air.

[0031] According to another characteristic of the invention, it ispossible to complete the process according to the invention by adding anadditional step consisting, after the sacrificial resin has beenremoved, in producing, on the upper face of the deformable membrane ormembranes, raised regions capable of modifying the moment of inertia ofthe surface of the deformable membrane so as to produce membranes withprogrammed deformation. This is because, as already mentioned, themodification in the capacitance of the microcomponent may arise frommodification in the spacing of the deformable membrane with respect tothe fixed membrane when the latter are subjected to a DC voltagecomponent. It is therefore possible, by virtue of this arrangement, toadapt the deformation of the deformable membrane, and therefore thevariation in the capacitance, to a variation in the DC component towhich the microcomponent is subjected.

[0032] In practice, the raised features produced on the membranes mayadvantageously be longitudinal ribs.

[0033] In practice, the metals used to produce the various plates mayadvantageously be chosen from the group comprising, especially, copper,chromium, nickel and alloys including these metals. Different metals maybe used to produce the plates and the spacer ribbon. The choice of thevarious materials and of the various electrolysis conditions makes itpossible to accurately establish the internal stresses in the deformablemembrane.

[0034] By virtue of this characteristic, it is possible to give thedeformable membrane a cross section which varies over its length.

BRIEF DESCRIPTION OF THE FIGURES

[0035] The manner in which the invention is realized and the advantageswhich stem therefrom will become clearly apparent from the descriptionof the method of implementation which follows, supported by the appendedfigures in which:

[0036]FIG. 1 is a top view of a microcomponent produced according to theprocess of the invention, illustrated after the fixed plate has beendeposited;

[0037]FIG. 2 is a sectional view of FIG. 1 in the direction of thearrows II-II′;

[0038]FIG. 3 is a top view of the microcomponent according to theinvention illustrated after the anchoring grooves have been etched;

[0039]FIG. 4 is a sectional view of FIG. 3 in the direction of thearrows III-III′;

[0040]FIG. 5 is a top view of the microcomponent produced according tothe invention after the spacer ribbons have been deposited;

[0041]FIG. 6 is a sectional view of FIG. 2 in the direction of thearrows IV-IV′;

[0042]FIG. 7 is a top view of a microcomponent during productionaccording to the invention, after the sacrificial resin layer has beendeposited;

[0043]FIG. 8 is a sectional view of FIG. 7 in the direction of thearrows VIII-VIII′;

[0044]FIG. 9 is a top view of the microcomponent according to theinvention, illustrated after the deformable plate has been deposited;and

[0045]FIG. 10 is a sectional view of FIG. 9 in the direction of thearrows X-X′.

MANNER OF REALIZING THE INVENTION

[0046] As already stated, the invention relates to a process allowingmicrocomponents, such as microcapacitors or microswitches, to beproduced. By using electrolysis steps not requiring high temperatures,it is possible to implement this process of producing themicrocomponents either directly on quartz layers or directly onpre-existing integrated circuits.

[0047] Since the implementation of the process may be carried out onboth these types of support, the internal structure of the substrate onwhich the microcomponent according to the invention is produced will notbe described below.

[0048] Thus, as shown in FIG. 1, the first series of steps in theprocess consists in producing a fixed plate (1) on a substrate (2). Thesubstrate (2) may either be a quartz layer, if a microcomponent isproduced independently of an integrated circuit, or else the oxide layerobtained, for example, by PECVD (plasma-enhanced chemical vapordeposition), typically with a thickness of a few microns, which coversan integrated circuit.

[0049] To produce the fixed plate (1), the process starts with theproduction of a well (3) by etching into the oxide layer (2). Next, ametal growth sublayer (4), typically made of a copper/titanium alloy, isdeposited on the oxide layer (2) and the well (3).

[0050] The well produced has a shape encompassing both that of the fixedplate (1) and that of an extension (5) forming the voltage lead to thefixed plate (1). This voltage lead (5) may be extended by a pad (7)allowing it to be connected to the microcomponent.

[0051] Next, copper is deposited electrolytically on top of the growthsublayer (4). The metal used for the electrolysis is preferably copper,chosen for its low resistivity. The electrolysis is continued until thecopper layer (1) has a thickness sufficient to fill the well (3) made inthe oxide layer (2).

[0052] Next, as illustrated in FIGS. 1 and 2, the copper layer (1) isplanarized, allowing it to be given a surface finish with a very highflatness.

[0053] Next, a dielectric layer (6) is deposited, typically PECVDsilicon oxide or nitride. This oxide film (6) has the minimum thicknessfor preventing the phenomena of the deformable membrane bonding to thefixed plate (1). Interposing this oxide film (6) slightly increases thecapacitance of the capacitor which will be formed with the movableplate, if the dielectric constant of this film is greater than that ofair.

[0054] Next, as illustrated in FIGS. 3 and 4, the substrate layer (2) isetched by a conventional lithographic etching process making it possibleto create a peripheral anchoring groove (8, 9). In general, this groovehas a width (w) about twice the thickness of the future upper deformablemembrane. The depth (d) of this groove (8, 9) is at least one and a halftimes its width (w).

[0055] Next, a new growth sublayer (14) is deposited on the surface ofthe entire region of the microcomponent. By lithographic etching, thisgrowth. sublayer is covered except in the regions for producing theperipheral borders, so as to create growth regions for the electrolysisof the peripheral ribbon (see FIG. 5). This ribbon (10, 11) is thenformed by electrolysis. This ribbon (10, 11) may be made of a materialidentical to or different from that serving for the fixed plate (1) andfor the movable plate, depending on the result desired. In theembodiment illustrated, the ribbon (10) is located between the groove(8) and the boundary (13) of the fixed plate (1), but in certainsituations it is conceivable for this peripheral ribbon(10, 11) to bepartly embedded within the anchoring groove (8).

[0056] Moreover, the embodiment illustrated in FIG. 5 shows a peripheralribbon located on either side of the fixed plate (1) and consisting of asingle band (10, 11) on each side. If it is desired to produce severaldeformable membranes on top of a single common fixed plate (1), themetal ribbon may then be segmented into as many portions as necessary.

[0057] Next, as illustrated in FIGS. 7 and 8, a sacrificial resin (15)is deposited with a thickness, close to 2 micrometers, correspondingapproximately to the gap which will exist between the fixed plate (1)and the deformable membrane. Typically, this resin has a thicknessapproximately equal to that of the peripheral ribbon (10, 11). Ofcourse, this thickness value is given by way of example, and it may betailored according to the geometry associated with the applications. Thematerial used for this resin is among the materials conventionally usedin microelectronics.

[0058] Next, an electrolytic growth sublayer is deposited on top of thesacrificial resin layer (15), the exposed part of the ribbon (10, 11)and the anchoring grooves (8, 9). This layer is typically made oftitanium or chromium, chosen for its capability of adhering to silica.The process then continues with a lithographic step in order to coverthis growth sublayer and to leave it exposed only at the necessaryplaces on the sacrificial resin layer (15) and at the peripheral ribbon(10, 11) and anchoring groove (8, 9).

[0059] Next, an electrolysis step is carried out allowing the deformablemembrane (20) to be produced on top of the remaining growth sublayer.The choice of metal used depends on the stresses required for thestiffness of the membrane.

[0060] The process then continues with the removal of the sacrificialresin layer and of the growth sublayer covered beforehand by thelithographic etching.

[0061] As illustrated in FIG. 9, the movable membrane (20) has,vertically in line with the border of the fixed plate, indentations(21-24) defining an aperture for passage of the substances used fordissolving the resin. These indentations are especially useful ifseveral deformable membranes are placed side by side. In the case of amicrocomponent having only one movable membrane of large dimensions, itis then pierced by holes or indentations over a major part of itssurface in order to allow flow of the substances needed to dissolve thesacrificial resin.

[0062] Next, according to another characteristic of the invention, asecond resin is deposited on top of the deformable membrane. This secondresin may be exposed in certain characteristic regions in order toreveal certain regions of the deformable plate (20) itself. In theseregions, it is then possible by an additional electrolysis operation toform beams (25) whose stiffness is added to and combined with those ofthe deformable membrane. Thus, the moment of inertia of the surface ismodified, thereby modifying the membrane deformation law.

[0063] Advantageously, these additional beams (25) have a thicknesspossibly up to 30 microns and may be made, for example, of nickel. Next,the additional resin deposited on top of the deformable membrane (20) isremoved.

[0064] Of course, the various thicknesses and the dimensions and choiceof materials used depend on the technological constraints of the desiredapplications and are not limited to the valid ones described above.Thus, it is possible to obtain capacitors whose capacitance can varybetween a few tens of femtofarads and a few tens of picofarads.Depending on the application, the gap may be up to a few micrometers. Ofcourse, these values are given by way of example and they may varydepending on the types of components produced and on their applications.

[0065] It is apparent from the foregoing that the process according tothe invention has many advantages and especially:

[0066] the possibility of producing a capacitor of variable capacitance,whose gap, or the distance between its fixed plate and its variableplate, is particularly uniform, making the component very precise;

[0067] the possibility of using microcomponents such as a microswitchfor applications operating in the radio frequency range (from 100megahertz to 5 gigahertz), the use of low-resistivity metals, such ascopper or gold, proving to be advantageous with regard to insertionlosses of the metal/metal contact,

[0068] for applications in the high-frequency range (from 5 gigahertz to100 gigahertz), the use of a dielectric interlayer making it possible toobtain a capacitive contact, without any risk of causing bonding betweenthe movable membrane and the fixed membrane;

[0069] the possibility of producing a plurality of variable capacitorssharing the same fixed plate;

[0070] the use of relatively low temperatures, allowing the process tobe carried out directly on integrated circuits without any risk ofmodifying the functionalities of said circuits; and

[0071] the possibility of modifying the mechanical structure of thedeformable membrane in order to adapt its deformation law to the desiredapplication.

1. A process for fabricating electronic microcomponents, of the variablecapacitor or microswitch type, comprising a fixed plate (1) and adeformable membrane (20) which are located opposite each other, whichcomprises the following steps, consisting in: depositing a first metallayer on an oxide layer (2), said first metal layer being intended toform the fixed plate; depositing a metal ribbon (10, 11) on at leastpart of the periphery and on each side of the fixed plate (1), saidribbon being intended to serve as a spacer between the fixed plate (1)and the deformable membrane (20); depositing a sacrificial resin layer(15) over at least the area of said fixed plate (1); generating, bylithography, a plurality of wells in the surface of said sacrificialresin layer; depositing, by electrolysis, inside the wells formed in thesacrificial resin (15), at least one metal region intended to form thedeformable membrane (20), this metal region extending between sectionsof the metal ribbon (10, 11) which are located on each side of saidfixed plate (1); removing the sacrificial resin layer (15).
 2. Theprocess as claimed in claim 1, wherein the oxide layer (2) is depositedon an integrated circuit.
 3. The process as claimed in claim 1, whereinthe oxide layer (2) is made of quartz.
 4. The process as claimed inclaim 1, wherein the first metal layer intended to form the fixed plateis inserted into a recess (3) formed in the oxide layer (2).
 5. Theprocess as claimed in claim 1, wherein the first metal layer includes anextension (5) associated with a connection pad (6).
 6. The process asclaimed in claim 1, wherein the ribbon (10, 11) is present along theperiphery of the fixed plate (1), on two opposed sides of the latter. 7.The process as claimed in claim 1, wherein the ribbon consists of asuccession of individual segments.
 8. The process as claimed in claim 1,wherein the sacrificial resin layer (15) partly covers the peripheralribbon (10, 11).
 9. The process as claimed in claim 1, which alsoincludes a step consisting in etching the oxide layer in order to formone or more anchoring grooves (8, 9) intended to accommodate part of theperipheral ribbon (10, 11).
 10. The process according to claim 1, whichalso includes a step consisting in etching the oxide layer in order toform one or more anchoring grooves (8, 9) intended to accommodate partof the ends of the deformable membrane (20).
 11. The process as claimedin either of claims 9 and 10, wherein the anchoring groove (8, 9) has awidth (w) about twice the thickness of the deformable membrane (20). 12.The process as claimed in claim 11, wherein the anchoring groove (8, 9)has a depth (d) more than one and a half times its width (w).
 13. Theprocess as claimed in claim 1, which furthermore includes a stepconsisting, after the fixed plate has been deposited, in depositing afilm (6) of a dielectric, intended to prevent the deformable membranefrom bonding to said fixed plate.
 14. The process as claimed in claim 1,which furthermore includes a step consisting, after the sacrificialresin (15) has been removed, in producing, on the upper face of thedeformable membrane or membranes, raised regions (25) capable ofmodifying the moment of inertia of the surface of the deformablemembrane so as to produce membranes with programmed deformation.
 15. Theprocess as claimed in claim 14, wherein the raised features arelongitudinal ribs (25).
 16. The process as claimed in claim 1, whereinthe metal used both for producing the fixed plate and the deformablemembrane is chosen from the group comprising copper, chromium, nickeland alloys including these metals.
 17. A microcomponent of the variablecapacitor or microswitch type, comprising: a fixed metal plate (1)inserted into an oxide layer (2); at least two metal ribbons (10, 11)located peripherally and on each side of said fixed plate (1); at leastone deformable metal membrane (20) located opposite said fixed plate(20) and resting at its two ends on the two metal ribbons.
 18. Themicrocomponent as claimed in claim 17, which is located on the upperface of an integrated circuit.
 19. The microcomponent as claimed inclaim 17, wherein the deformable membrane (20) has, on its upper face,raised features (25) intended to modify its moment of inertia of thesurface.
 20. The microcomponent as claimed in claim 17, wherein thefixed plate (1) is covered with a film (6) of a dielectric intended toprevent the deformable membrane (20) from bonding to the fixed plate(1).
 21. The microcomponent as claimed in claim 17, which includesanchoring grooves (8, 9) formed in the oxide layer (2) and locatedoutside the fixed plate, said grooves (8, 9) being filled with a portionof the spacer ribbons (10, 11) and/or with the deformable membrane (20).22. The microcomponent as claimed in claim 17, wherein the metalconstituting the fixed plate and the movable membrane is chosen from thegroup comprising copper, nickel and chromium.
 23. The microcomponent asclaimed in claim 17, wherein the cross section of the deformablemembrane (20) varies over the length of said deformable membrane.